The present invention relates to plate-like composite ferrite fine particles for magnetic recording having a large magnetization, an appropriate coersive force, a large anisotropy field and an excellent temperature dependence of the coercive force (temperature stability), and a process for producing such plate-like composite ferrite fine particles.
Recently, as described, for example, in Japanese Patent Application Laid-Open (KOKAI) No. 55-86103 (1980), ferromagnetic non-acicular particles have been demanded as a magnetic recording material, in particular, as a perpendicular magnetic recording material.
Generally, plate-like ferrite containing Ba particles are known as ferromagnetic non-acicular particles.
A method of autoclaving an aqueous alkaline suspension containing Ba ions or Sr ions and Fe(III) by using an autoclave as a reaction apparatus (this method is hereinafter referred to as "autoclaving method") is hitherto known as a process for producing plate-like ferrite.
Plate-like ferrite fine particles for magnetic recording are required to have as small a particle size as possible, to have an appropriate coercive force and a large magnetization as the magnetic properties, and to be so excellent in the temperature dependence of the coercive force (temperature stability). This will be explained in more detail in the following.
Firstly, the particle size of plate-like ferrite fine particles suitable for magnetic recording is required to be as fine as possible, in particular, to be not greater than 0.3 .mu.m. This fact is described, for example, in Japanese Patent Application Laid-Open (KOKAI) No. 56-125219 (1981) "The utility of the perpendicular magnetic recording to longitudinal recording become manifest in the region in which the recording wavelength is not more than 1 .mu.m. In order to sufficiently record and reproduce in this wavelength region, the crystalline particle diameter of the ferrite is preferably substantially not greater than 0.3 .mu.m. However, if the particle diameter becomes smaller than about 0.01 .mu.m, the desired coercive force tends to be not exhibited. Therefore, a crystalline particle diameter of 0.01 to 0.3 .mu.m is appropriate".
Secondly, with respect to the magnetic properties, the plate-like ferrite containing Ba fine particles are generally required to have a coercive force of about 300 to 2,000 Oe, and in order to reduce the coercive force of the plate-like ferrite containing Ba particles produced in the autoclaving method to an appropriate coercive force, it has been proposed to substitute a part of Fe.sup.3+ in the ferrite by Co(II), Ti(IV) or the ions of a divalent metal M(II) such as Mn and Zn.
The magnetization of plate-like ferrite containing Ba particles must be as large as possible, as is described in Japanese Patent Application Laid-Open (KOKAI) No. 56-149328 (1981), ". . . the magnetoplumbite ferrite which is used as a magnetic recording material is required to have the greatest possible saturation magnetization . . .".
The coercive force of plate-like ferrite containing Ba particles generally has a tendency of increasing with the rise of temperature, as is seen from FIG. 4 on page 1123 of IEEE TRANSACTIONS ON MAGNETICS MAG-18 No. 6. At the time of recording and reproduction, the temperatures of a magnetic head and a magnetic medium are raised with the reciprocal friction, so that the writing capacity for recording of the magnetic head is lowered, while the coercive force of the medium containing the plate-like ferrite containing Ba particles as magnetic particles is increased with the rise of temperature, resulting in the drop of output or deterioration of the overwrite characteristic. It is, therefore, necessary in order to enable record-writing in spite of the lowered writing capacity of the magnetic head that the coercive force of the medium scarcely varies or rather has a tendency to lower with the rise of temperature. Namely, it is necessary that the temperature dependence of the coercive force (hereinafter referred to as "temperature stability") of plate-like ferrite containing Ba particles is not more than 2.0 Oe/.degree.C. at a temperature of -20.degree. to 120.degree. C.
Plate-like ferrite fine particles which have as small a particle size as possible, a large magnetization, an appropriate coercive force and an excellent temperature stability are now in the strongest demand. In the above-described autoclaving method, various kinds of ferrite particles precipitate depending on the selected reaction conditions. The precipitated particles ordinarily have a shape of a hexagonal plate, and the particle properties such as particle size distribution and average particle diameter, and the magnetic properties such as coercive force, magnetization and temperature stability are different depending upon the conditions under which the ferrite particles are produced.
For example, in plate-like composite Ba ferrite fine particles containing Co(II) - Ti(IV) obtained by autoclaving method while substituting a part of Fe(III) in the ferrite by Co(II) and Ti.sup.4+ in order to reduce the coercive force to an appropriate coercive force and heat-treating the thus-obtained fine particles, the coercive force reducing-effect of Co(II) - Ti(IV) is large. As a result the necessary amount of Co(II) and Ti(IV) added to appropriately control the coercive force is so small as not to greatly lower the magnetization, which is maintained at a relatively large value such as about 50 to 60 emu/g. However, the temperature stability is +2.5 Oe/.degree.C. to 6.0 Oe/.degree.C. at a temperature of -20.degree. to 120.degree. C., and the coercive force of the plate-like composite Ba ferrite particles containing Co(II) and Ti(IV) has a tendency of increasing with the rise of temperature, as described above. This phenomenon is presumed from FIG. 1 on page 1459 of JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS Nos. 15 to 18 (1980).
As a method of improving the temperature stability of plate-like composite ferrite fine particles containing elements such as Co(II) and Ti(IV) for reducing the coercive force, methods disclosed in, for example, Japanese Patent Application (KOKAI) Laid-Open Nos. 61-152003 (1986) and 62-132732 (1987) are conventionally known.
Japanese Patent Application (KOKAI) Laid-Open No. 61-152003 (1986) discloses a method of heat-treating the plate-like composite ferrite fine particles containing elements such as Co(II) and Ti(IV) for reducing the coercive force at a temperature of 300.degree. to 700.degree. C. in a reducing atmosphere. This method is disadvantageous in that the coercive force after heat-treating is increased to more than twice the value before heat-treating, thereby making the appropriate control of the coercive force difficult.
Japanese Patent Application (KOKAI) Laid-Open No. 62-132732 (1987) discloses a method of forming plate-like composite ferrite particles having uniform particle shape such that the average particle diameter is not more than 1.0 .mu.m, the thickness in the direction of the c-axis is not more than 0.2 .mu.m and a plate ratio (average diameter of plate surfaces/thickness in the direction of c-axis) is not less than 5. This method is disadvantageous in that the particle shape is regulated in order to improve the temperature stability.
Plate-like ferrite containing Ba particles are also required to have a large anisotropy field so as to enable high-density recording without lowering the output even in a high-frequency band. This fact is described, for example, on pages 67 to 68 of HIGH-DENSITY MEMORY TECHNIQUE AND MATERIALS (1984), published by K. K. CMC, "FIGS. 2, 3 and 12 show the characteristics between the output and the wave-length obtained by recording/reproducing with a ring head by using a Co-Cr single-layer medium having a large perpendicular anisotropy field (Hk). An excellent high-density recording characteristic such as D.sub.50 =135 KBPI is obtained . . . ".
The establishment of a method of producing plate-like Ba ferrite fine particles having as small a particle size as possible, an appropriate coercive force, a large magnetization, a more large anisotropy field and an excellent temperature stability without any regulation in the particle shape has, therefore, been strongly demanded.
As a result of various studies on a process for producing plate-like composite Ba ferrite fine particles containing Co.sup.2+ and Ti.sup.4+, having as small a particle size as possible, an appropriate coercive force, a large magnetization, a more large anisotropy field and an excellent temperature stability that the coercive force scarcely varies with the rise of temperature without any regulation in the particle shape, it has been found that by autoclaving a suspension of an alkaline iron (III) hydroxide containing Ba ions as well as a Ni Compound, a Co compound and a Ti compound in molar ratios defined by 0.02.ltoreq.Ni/Fe .ltoreq.0.10, 0.01.ltoreq.Co/Fe .ltoreq.0.10 and 0.01.ltoreq.Ti/Fe .ltoreq.0.10, in the range of but.0.03.ltoreq.(Ni+Co)/Fe .ltoreq.0.14 and Ti/(Ni+Co)&lt;1, at a temperature of 90.degree. to 300.degree. C. to produce the plate-like composite Ba ferrite fine particles; suspending the thus obtained fine particles in an aqueous solution containing zinc and having a pH of 4.0 to 12.0; filtering out the fine particles with a Zn hydroxide precipitated on the surfaces thereof; and after washing with water and drying, calcining (heat-treating) the thus obtained particles in the temperature range of 600 to 900.degree. C., the thus obtained plate-like composite Ba ferrite particles containing Ni, Co and Ti in the molar ratios defined by 0.02.ltoreq.Ni/Fe .ltoreq.0.10, 0.01.ltoreq.Co/Fe .ltoreq.0.10, and 0.01.ltoreq.Ti/Fe .ltoreq.0.10, but 0.03.ltoreq.(Ni+Co)/Fe .ltoreq.0.14 and Ti/(Ni+Co)&lt;1, and having a concentration gradient of Zn with 0.2 to 5 wt % (calculated as Zn) through the particle in the form of a solid solution, have an appropriate coercive force, a large magnetization, a large anisortopy field of not less than 4.0 KOe and a temperature stability of -0.5 Oe/.degree.C. to +2.0 Oe/.degree.C. in the temperature range of -20.degree. C. to 120.degree. C. The present invention has been achieved on the basis of this finding.